This invention relates to high intensity arc discharge lamps and more particularly to high intensity arc discharge metal halide lamps having high efficacy.
Due to the ever-increasing need for energy conserving lighting systems that are used for interior and exterior lighting, lamps with increasing lamp efficacy are being developed for general lighting applications. Thus, for instance, arc discharge metal halide lamps are being more and more widely used for interior and exterior lighting. Such lamps are well known and include a light transmissive arc discharge chamber sealed about an enclosed a pair of spaced apart electrodes, and typically further contain suitable active materials such as an inert starting gas and one or more ionizable metals or metal halides in specified molar ratios, or both. They can be relatively low power lamps operated in standard alternating current light sockets at the usual 120 Volts rms potential with a ballast circuit, either magnetic or electronic, to provide a starting voltage and current limiting during subsequent operation.
These lamps typically have a ceramic material arc discharge chamber that usually contains quantities of metal halides such as CeI3 and NaI, (or PrI3 and NaI) and T1I, as well as mercury to provide an adequate voltage drop or loading between the electrodes, and also an inert ionization starting gas. Such lamps can have an efficacy as high as 145 LPW at 250 W with a Color Rendering Index (CRI) higher than 60, and with a Correlated Color Temperature (CCT) between 3000K and 6000K at 250 W.
Of course, to further save electric energy in lighting by using more efficient lamps, high intensity arc discharge metal halide lamps with even higher lamp efficacies are needed. The efficacy of a lamp is affected by the shape of the arc discharge chamber therein. If the ratio between the distance separating the electrodes in the arc discharge chamber to the diameter of the chamber is too small, such as being less than four, the relative abundance of Na between the arc and the chamber walls leads to a lot of absorption of generated light radiation by such Na due to its absorption lines near the peak values of visible light. Also, if this ratio is less than five, the lamp operated with its length positioned horizontally results in the arc established in the arc discharge chamber substantially bending upward due the buoyancy of its vaporized chamber constituents. This upward bending of the arc brings it nearer to the wall of the arc discharge chamber near the peak of the bend, and so raises the temperature of the chamber wall in that vicinity. Such temperature increases can accelerate reactions of some of these vaporized constituents in the chamber and the elevated temperature portions of the chamber wall to thereby ultimately result in the destruction of the wall integrity, and so reduce the operating life of the lamp when operated horizontally.
On the other hand, if the ratio between the distance separating the electrodes in the chamber to the diameter of the chamber is too great, such as being greater than five, initiating an arc discharge in the arc discharge chamber is difficult because of the relatively large breakdown distance between the electrodes. In addition, such lamps perform relatively poorly when the long dimension thereof is oriented vertically during operation in exhibiting severe colors segregation as the different buoyancies of the lamp content constituents cause them to segregate themselves from one another to a considerable degree along the arc length, and reduced efficacy.
Increased pressures in the arc discharge chamber of either the mercury or the starting gas constituents therein, although having some helpful effects on such color segregation and on efficiency, also has detrimental aspects. Increased starting gas pressure is usually insufficient by itself to achieve these goals, and increased mercury pressure leads to needing to generate high operating voltages between the chamber electrodes and also to substantial discharge arc bending bringing the arc closer to the wall of the chamber to thereby shorten the operational duration of the lamp. Thus, there is a desire for arc discharge metal halide lamps having higher efficacies and better color performance.